A typical portable electronic device, such as a digital camera, includes a DC-DC converter. The DC-DC converter raises or lowers an input voltage from, for example, a lithium ion battery or a dry battery, generates an output voltage at a desired voltage level, and supplies the output voltage as a power supply to an electronic component (load), such as a semiconductor device, in the electronic device.
To generate the desired output voltage by controlling “on” and “off” states of a switching element and raising or lowering the input voltage from the battery, the DC-DC converter includes, for example, an error amplifier and a comparator. The DC-DC converter causes the error amplifier and the comparator to determine whether the output voltage is maintained at the desired voltage level. Further, the DC-DC converter causes the error amplifier and the comparator to set the lengths of time during which the switching element may be in the “on” and “off” states and to control the “on” and “off” states of the switching element so that the output voltage may reach the desired voltage.
For example, Japanese Patent Application Laid-Open Publication No. 2004-040858 discusses a DC-DC converter including a short-circuit protection circuit for detecting a change in an output voltage and stopping the operation of the DC-DC converter when a load to which the output voltage is supplied is electrically disconnected from the DC-DC converter.
For example, the error amplifier and the comparator of the DC-DC converter operate using the input voltage from the battery provided in the electronic device. The output voltage is generated based on the input voltage. The input voltage is requested to be a given voltage, for example, 2.5 V or more so that the error amplifier and the comparator may operate as desired. The battery is increasingly discharged as the electronic device is used for a longer time. When the input voltage falls below the given voltage, the error amplifier and the comparator may not operate as desired. As a result, the DC-DC converter may neither generate the desired output voltage nor output the voltage to each load.
In a low voltage system using a dry battery, a nickel metal hydride (NiMH) battery, or the like, the lowest input voltage is 1.8 V, for example. As a DC-DC converter that may operate with such a low input voltage, a self power supplying DC-DC converter that raises an input voltage by itself and generates an operation power supply voltage not less than a given voltage, and supplies the generated operation power supply voltage to, for example, an error amplifier and a comparator is known.
FIG. 1 is a circuit diagram illustrating a typical self power supplying DC-DC converter 50. The DC-DC converter 50 includes an inductor L1, a smoothing capacitor C1, a first transistor Tr1, a second transistor Tr2, and a control circuit 51 for complementarily controlling the “on” and “off” states of the transistors Tr1 and Tr2.
When the transistor Tr1 is turned on and the transistor Tr2 is turned off in the DC-DC converter 50, the energy corresponding to an input voltage VIN (battery voltage) is stored in the inductor L1. When the transistor Tr1 is turned off and the transistor Tr2 is turned on, the energy stored in the inductor L1 is discharged to an output terminal Ta through the transistor Tr2 and smoothed by the smoothing capacitor C1. The voltage direction in the inductor L1 during the energy discharge is the same as the direction of the input voltage VIN (battery voltage). Therefore, an output voltage VO raised higher than the input voltage VIN is generated.
For example, the control circuit 51 includes an error amplifier 52 and a pulse width modulation (PWM) comparator 53 that receive the output voltage VO as an operation power supply voltage VCC. For example, the error amplifier 52 and the PWM comparator 53 generate a PMW signal SG1 based on the generated output voltage VO. The control circuit 51 controls the “on” and “off” states of the transistors Tr1 and Tr2 based on the PWM signal SG1 so that the output voltage VO may become a given target voltage.
Further, the control circuit 51 receives the generated output voltage VO as the operation power supply voltage VCC and a voltage comparator 54 included in the control circuit 51 compares the output voltage VO (operation power supply voltage VCC) with a given reference voltage Vk. When the voltage comparator 54 determines that the operation power supply voltage VCC is lower than the reference voltage Vk, which implies that, due to the discharge, the input voltage VIN (battery voltage) becomes so low that the error amplifier 52, the PWM comparator 53, and the like may not be operated as desired, the “on” and “off” states of the transistors Tr1 and Tr2 are controlled based on an oscillation pulse signal SP from a ring oscillator 55 and the output voltage VO is raised. Thus, even when the discharged amount of the battery increases and the battery voltage becomes lower than the reference voltage Vk, the DC-DC converter 50 may cause the error amplifier 52, the PWM comparator 53, and the like to operate as desired and may generate the desired output voltage VO.
However, when a load to which the output voltage VO is supplied is electrically disconnected from the DC-DC converter 50 for some reason, a typical protection circuit may not be provided and problems described below may occur.
When the load is electrically disconnected from the DC-DC converter 50 for some reason, the output voltage VO becomes lower and the voltage comparator 54 wrongly determines that the input voltage VIN (battery voltage) would no longer be high enough to operate the error amplifier 52, the PWM comparator 53, and the like as desired. As a result of the wrong determination, the “on” and “off” states of the transistors Tr1 and Tr2 are controlled based on the oscillation pulse signal SP from the ring oscillator 55 and the output voltage VO is raised. Thus, a current may continue to flow toward the load even though the load is electrically disconnected.